Electrical lampholders generally comprise a securing arrangement by which a lamp may be secured in the lampholder. At least one contact, in the form of a biased plunger, for example, is usually provided to deliver electrical power to the lamp. Some lampholders also include an on/off switch mechanism by which electrical power to the lamp can be switched on and off by a user. In most known lampholders, the switch mechanism is arranged such that it can be switched on and off irrespective of whether or not there is a lamp in the lampholder. However, it has recently been recognized that this can give rise to a potentially dangerous situation where for example the switch mechanism is switched on without a lamp or other device in the lampholder. Under these circumstances, the electrical power delivery contact or contacts of the holder will be exposed, and this can result in electrocution of a person who inadvertently touches the contact.
One solution to this potentially dangerous situation is disclosed in British Patent Application GB-A-2,207,818. This patent specification describes a bayonet bulb holder in which a so-called shutter element cooperates with a transverse bar which forms part of a switch mechanism to prevent the bulb holder from being switched on unless a bulb is inserted in the holder. A peg on the shutter element engages with a slot in the transverse bar to inhibit movement of the bar. When a bulb is inserted in the holder, bayonet lugs thereon effect rotation of the shutter element, causing the peg to move away from the slot in the transverse bar and thereby releasing the bar to be switched to its "on" position. The transverse bar and the peg are further arranged to prevent rotation of the shutter element when the bar is in its "on" position. In this way, removal of a bulb is prevented unless the switch mechanism is in its "off" position.
While the bulb holder disclosed in GB-A-2,207,818 provides a solution to the problem of electrically live contacts being exposed, it is a limited solution in that there is still one potentially dangerous situation which appears to have been overlooked. This situation could arise when a bulb burns out in the bulb holder while the switch mechanism of the bulb holder is on. Under these circumstances, a person wishing to change the bulb, and who is unaware or has forgotten that the switch mechanism must be switched off before the bulb can be removed, may try to force the bulb out of the bulb holder. This may result in the bulb breaking and thereby exposing live wires within the bulb.
We have previously addressed this problem of live contacts being exposed in our as yet unpublished British Patent Application No. 8914956.1, the teachings of which are incorporated herein by reference. The lampholder disclosed in patent application includes an interlock tube having an arm extending therefrom, which arm co-operates with a slot in a pushbar to prevent movement of the pushbar unless a bulb is inserted in the lampholder. As a bulb is inserted in the lampholder the interlock tube is rotated, thereby moving the arm out of the slot and releasing the pushbar for movement between its "on" and "off" positions.
Our previously proposed lampholder also incorporates a means by which the pushbar is moved automatically to its "off" position as a bulb is removed. A camming surface is provided on the pushbar in association with the slot. The camming surface is arranged such that, as a bulb is removed rotation of the interlock tube causes the arm to act against the camming surface, thereby urging the pushbar to its off position. Thus, our previously proposed lampholder, in addition to providing a means by which the pushbar is prevented from being switched to its "on" position unless there is a bulb in the holder, also provides a means by which the pushbar is moved automatically to its "off" position as a bulb is removed from the holder.
Our previously proposed lampholder provides a further solution to the problem of exposed live contacts, but is best suited for use with bayonet-type connections because the angular movement associated with bayonet-type connections is well suited to importing angular movement to the interlock tube. However, our previously proposed lampholder and the bulb holder described in the abovementioned GB-A-2,207,818 are not particularly well suited for use with so-called edison screw connections in which a screw threaded cap co-operates with a similarly threaded member on the lampholder.